The present invention relates to a method for partially combusting coal which contains at least about 1 wt.% sulfur wherein a major portion of the sulfur content of the coal is retained in the solid effluents.
Although coal is by far our most abundant fossil fuel, there are serious problems associated with its use which has prevented coal from reaching its full commercial exploitation. Examples of some such problems include problems in handling, waste disposal, and pollution. As a result, oil and natural gas have acquired a dominant position throughout the world from the standpoint of fuel sources. This, of course, has led to depletion of proven petroleum and natural gas reserves to a dangerous level from both a worldwide energy, as well as an economic point of view.
One area in which it is desirable to replace petroleum and gas with coal as an energy source, is in industries where coal can be burned in combustion devices such as boilers or furnaces. Owing to environmental considerations, the gaseous effluents resulting from the combustion of coal in these devices must be substantially pollution free--especially with respect to oxides of sulfur (SO.sub.x) and nitrogen (NO.sub.x). One method conventionally employed for controlling SO.sub.x emissions is by flue gas scrubbing. The cost of flue gas scrubbing is prohibitive on small installations and excessive on large scale operations. There are also serious operating problems associated with flue gas scrubbing.
A two stage coal combustion process for minimizing SO.sub.x emissions is disclosed in U.S. Pat. No. 4,285,283 which is incorporated herein by reference. The process requires a coal having an organic calcium to sulfur ratio of at least 2 to 1 for coals containing less than 1 wt.% sulfur and a ratio of at least 1 to 1 for coals containing greater than 1 wt.% sulfur. The first stage requires combustion in the presence of an oxidizing agent at an equivalence ratio of at least 1.5. The second stage requires combustion of the gaseous effluents under oxidizing conditions at a temperature from about 1,000.degree. C. to about 1,500.degree. C.
Although such processes have met with varying degrees of success in a commercial environment, there is still a need in the art for alternative combustion processes for minimizing SO.sub.x emissions without sacrificing fuel utilization to an undesirable degree.